This invention relates to a medical diagnostic ultrasound transducer system and method for harmonic imaging. In particular, a transducer system providing increased bandwidth for imaging with harmonic echoes from tissue, fluid or added contrast agents is provided.
Acoustic energy is transmitted into a patient at fundamental transmit frequencies. Acoustic energy is reflected off of tissue, fluid or other structures within the patient. The reflections include energy at the fundamental frequency band as well as energy generated at harmonic frequencies of the fundamental frequency band. The transducer converts the acoustic energy into an electrical signal.
Transducer bandwidth may limit the actual response, reducing the information content at harmonic or other frequencies. Manufacturing transducers with a 6 dB bandwidth or better exceeding 80% of the desired frequency range is difficult and expensive. For harmonic imaging, a 6 dB bandwidth exceeding 100 or 140% is preferably provided. For example, the transducer may transmit energy in a 3 to 5 MHz frequency range and receive information of interest in the 6 to 9 MHz range.
Larger bandwidth transducers are generally desirable for any type of ultrasonic diagnostic imaging. For example, transducers with a wide bandwidth are used for obtaining information at different fundamental frequencies during a same or different imaging sessions. Various techniques have been suggested for providing wide bandwidth transducers. For example, T. R. Gururaja et al in xe2x80x98Medical Ultrasonics Transducers With Switchable Frequency Bands Centered about f0 and 2f0xe2x80x99, 1997 IEEE Ultrasonic Symposium, pp. 1659-1662, disclose an electrostrictive transducer element using two layers. A selected bias is applied to one layer, and a transmit waveform is applied to an electrode between the two layers for wide bandwidth transmission. As another example, J. Hossack et al. in Improving the Characteristics of A Transducer Using Multiple Piezoelectric Layers, IEEE Transactions On Ultrasonics, Ferroelectrics and Frequency Control, Vol. 40, No. 2, March 1993, disclose a two-layer piezoelectric single element transducer. A different waveform is applied to each of the layers on transmit, and phasing or delays are applied to signals from one of the layers relative to another layer on receive. As another example, different materials in a single layer transducer element may be used to extend the frequency range of the transducer.
U.S. Pat. No. 5,957,851, the disclosure of which is incorporated herein by reference, discloses an ultrasound transducer with multiple piezoelectric layers for use in harmonic imaging. Diodes or a transistor is used to isolate one layer from the other during transmit or receive. The same transducer is used to transmit at a fundamental frequency and receive at a harmonic frequency. For this passive switching system, the same transmit and receive processing is performed for each layer when each layer is being used.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, the preferred embodiments described below include a method and transducer system for harmonic imaging. At least one transducer element is provided. The transducer element comprises two stacked piezoelectric layers. The layers are stacked in the height or thickness direction which is perpendicular to an elevation-azimuth plane (i.e. azimuth is X direction, elevation is Y direction and range is Z direction). Information from each of the layers is independently processed during one of a transmit event, a receive event, and both of transmit and receive events. Information from the transducer element is provided to a filter. The filter isolates harmonic information for imaging. By providing a multi-layer transducer element with independent processing for each layer, a wide bandwidth transducer for harmonic imaging is provided. The null associated with most transducers at the second harmonic of a fundamental frequency is removed or lessened.
Further aspects and advantages of the invention are described below in conjunction with the preferred embodiments.